1. Field of the Invention
The present invention relates generally to lithography systems. More particularly, this invention relates to cleaning a reticle during use in a lithography system.
2. Related Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer. While this description is written in terms of a semiconductor wafer for illustrative purposes, one skilled in the art would recognize that this description also applies to other types of substrates known to those skilled in the art.
During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image projected onto the surface of the wafer by an exposure system located within a lithography system. The exposure system includes a reticle (also called a mask) for projecting an image onto the wafer. The exposure system also includes an illumination system, a projection optics system, and a wafer alignment stage.
Particulate contamination on the reticle can be imaged on each pattern on the wafer. As the particle is not part of the desired pattern, the reticle image produced contains a defect (the image of the particle). In many cases, this defect can cause the functional failure of every pattern on every wafer printed with the contaminated reticle. As shorter and shorter wavelengths are employed to improve the lithography process and improve the capability to print finer features, the system becomes more sensitive to smaller particles. These are more difficult (or currently impossible) to detect on the reticle prior to its installation in the lithography tool. In addition, it is more difficult (if not impossible) to maintain the reticle environment clean enough to prevent the deposition of such small particles on the reticle.
While exposure optics are used in the case of photolithography, a different type of exposure apparatus can be used depending on the particular application. For example, x-ray, ion, electron, or photon lithographies each can require a different exposure apparatus, as is known to those skilled in the art. The particular example of photolithography is discussed here for illustrative purposes only.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the image features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those image features projected onto the wafer during exposure. This patterned layer is then used to remove or further process exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the wafer.
Step-and-scan technology works in conjunction with a projection optics system that has a narrow, typically rectangular imaging slot called the exposure field. Rather than expose the entire wafer at one time, individual fields are scanned onto the wafer one at a time. This is done by moving the wafer and reticle simultaneously such that the imaging slot is moved across the field during the scan. The wafer stage must then be asynchronously stepped between field exposures to allow multiple copies of the reticle pattern to be exposed over the wafer surface. In this manner, the quality of the image projected onto the wafer is maximized. While using a step-and-scan technique generally assists in improving overall image quality, image distortions generally occur in such systems due to imperfections within the projection optics system, illumination system, and the particular reticle being used. An exemplary step-and-scan lithography system is the Microscan II, manufactured by Silicon Valley Group, Inc., San Jose, Calif.
The illumination system of a lithographic system includes a light source. Excimer lasers are one such light source and operate at several characteristic wavelengths ranging from vacuum ultraviolet light to greater than 400 nanometers (nm) depending on the gas mixture used. By shortening the wavelength of the light, the resolution of the projection system is improved. Thus, in a lithography system, it is desirable to utilize a light source with wavelengths within the vacuum ultraviolet range, i.e., below 200 nm.
As shorter wavelength light sources are used in lithography, organic contamination in the exposure area of the lithography system becomes a greater problem. It is well known that organic contaminates have high optical absorption coefficients at shorter wavelengths, particularly at 157 nm and below. A 1 nm film of organic contaminant belonging to the alkane group will drop the optical transmission at 157 nm by 1%. Further, an acetone residue left on the surface of a calcium fluoride optical element reduces the transmission by 4% at 157 nm. (See, T. M. Bloomstein et al., Optical Materials and Coatings at 157 nm, 3676 S.P.I.E. Proceedings 342-9 (1999); which is incorporated herein by reference). Optical intensity is an important issue as the number of optical elements increases in a lithography system. It is for this reason that organic contamination can be detrimental to optical elements in 157 nm and below lithography systems.
Sources of organic contamination within a lithography system include out-gassed products from polymer materials and solvents used for degreasing tool parts, for example. Extremely low levels of organic contamination are critical for the exposure path in the lithography system, and an active purge system and strict material selection are required for those areas of the system associated with this path.
Therefore, what is needed is a technique of removing particle contamination of the mask over an extended period of time.
The present invention makes practical maintaining near zero particle contamination of the mask over an extended period of time by repetitive cleaning of the mask during the actual exposure process. The repeated cleaning shortens the effective amount of time that the mask is exposed to contamination, making realistic levels of environmental control (Class 1 to Class 10) consistent with a near zero particulate requirement.
The present invention utilizes a cleaning system in which the reticle is passed underneath a delivery device using a step and scan method. In one embodiment, the delivery device remains stationary. The delivery device transports a gas, which becomes ionized before being directed onto the mask surface of the reticle. The ionized gas neutralizes electro-static attraction between the mask and particulates, thereby xe2x80x9cblowing offxe2x80x9d the particulates. The ionized gas and particulates are then transported away from the mask surface of the reticle by a contaminant collector. A positive or negative charge can be applied to the contaminant collector to better promote collection of particulate contamination from the mask.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.0